The invention concerns intervertebral disc prostheses.
An intervertebral disc prosthesis intended to replace an intervertebral disc is known, for example, from the document EP-O,277,282. It comprises two plates which are able to bear against the vertebral plates of the vertebrae adjacent to the disc which is to be replaced, and a cushion which is interposed between the plates. The cushion comprises a compressible body and a chamber enclosing a liquid. This prosthesis has the disadvantage that the compressible body is susceptible to wear, which in the long term modifies the mechanical behavior of the prosthesis. Moreover, its friction against the plates causes erosion of particles which can migrate within the patient""s body.
U.S. Pat. No. 6,052,992 to Eroshenko discloses a structure for selectively accumulating and dissipating energy. Referring to FIG. 1, the structure includes a porous capillary solid matrix 11 and a liquid 13 that surrounds the porous capillary matrix. The matrix is lyophobic relative to the liquid, i.e., the surface tension is such that the liquid is non-wetting when it comes into contact with the surface of the matrix. The liquid 13 surrounding the porous capillary matrix 11 is selected to define a solid/liquid separation surface having an area that varies isothermally and reversably as a function of the external pressure to which the structure is subjected. As such, the liquid can only be forced into the pores or capillaries when sufficient compression is exerted on the structure. In FIG. 3a, the liquid 13 is present at the entrance to a capillary passage 12 formed in the porous capillary solid matrix 11. The liquid is unable to penetrate into the passage 12, however, forming a meniscus 15. Referring to FIG. 3b, when compressive forces are applied to the structure, the liquid is forced to penetrate into the capillary passage 12. When the pressure applied to the structure is released, spontaneous expansion is obtained to enable a return from the state shown in FIG. 3b to the state shown in FIG. 3a. 
It is an object of the invention to make available a prosthesis whose mechanical behavior is identical over a long period of time, and which reduces the risks of particles being released in the patient""s body.
With a view to achieving this object, the invention provides an intervertebral disc prosthesis comprising a compressible cushion having a body made of a material, and a liquid which is able to come into contact with the body, in which the liquid and the material are such that the liquid does not wet the body.
Given that the liquid does not wet the body, the forced contact of the liquid with the body requires a certain energy. This energy is restored in its entirety when the liquid is allowed to separate again from the body. A sort of spring effect is produced in this way. This effect is all the more appreciable as the surface of contact of the liquid with the body is great. The prosthesis can thus take up very substantial stresses, deforming under compression or flexion, and can then produce the inverse deformation by restoring the stored energy. Such a prosthesis can store a considerable quantity of energy in a small volume. In addition, the movement of the liquid in contact with the body does not generate any wear: the prosthesis is thus able to present the same mechanical behavior over a very long period of time. Moreover, the absence of friction between solid parts practically eliminates any risk of release of particles in the body.
The spring effect will be very great in the presence of a porous body, even one of very limited volume.
If the body has cavities communicating with each other along very long networks, the path to be traveled by the liquid upon entry and exit is particularly long. The result of this is that the liquid dissipates some of the energy on account solely of its circulation in the body. Consequently, the whole arrangement can constitute a partial damper. The combination of the spring effect and the damping effect thus means that for the prosthesis the curve representing the compression stress as a function of the variation in length of the prosthesis on its main axis has a hysteresis form, like a healthy natural intervertebral disc. The prosthesis thus comes very close to the mechanical behavior of a real disc.
The prosthesis advantageously comprises two plates which extend either side of the cushion, the body being fixed to at most one of the plates.
Other characteristics and advantages of the invention will become more apparent from the following description of a preferred embodiment which is given as a nonlimiting example.